@ariziragoran

You need to setup two different virtual hosts in Nginx, in one of them the server_name must be mysite.com and points to the directory where index.html for the desktop site is located use root directive for this.

And the other virtual host should have m.mysite.com in its server_name and its root directive should point to the other directory where the other index.html is located.

Check this for more information about how to setup nginx virtual hosts.

Hope this helps you.